I'm entering this as a bug because it feels like a very significant regression to me, in my usage pattern.
I add quite a few books, and "add by ISBN" seldom works for me. It was a minor nuisance but at least when it did fail, I used to get a message on my screen immediately that said "We were unable to import your book:
You can add the book manually by clicking the link below."
Last night, I was perplexed when 2 attempts to add books by ISBN failed with ZERO visible sign of failure. In one case, I tried 3 times, the other twice, because I did not know what was happening.
Then, just now, I noticed the red notification dot on my profile pic and saw two notifications for the failed imports. Because these notifications are now 10 hours old, I have to search the ISBNs to find out which books I was trying to add.
The old error message was much more visible and made it easier for me to manually add the book while the relevant external info pages were sill open, I hope some thought can be given to restoring that behaviour. Although it is probably less likely to happen going forward as I have bookmarked the "add manually" page to avoid wasting time trying to import by ISBN first.